Musnad Ahmad 1153:1068

Yazid ibn Harun narrated to us, Muhammad ibn Ishaq informed us, from Abd al-Rahman ibn al-Qasim ibn Muhammad, from his father, from Aisha, who said: "She is only Sahlah bint Suhayl, and the Messenger of Allah (peace and blessings of Allah be upon him) commanded her to perform ghusl for every prayer. When that became difficult for her, he commanded her to combine the Dhuhr and the Asr with one ghusl, and between the Maghrib and the Isha with one ghusl, and that she should perform ghusl for the morning prayer."